1. Field of the Invention
The present invention relates to the field of analytical chemical instrumentation utilizing optical sensors and in particular to integrated optical-chemical analytical instrumentation used in the fields of chemical, biochemical, biological or biomedical analysis, process control, pollution detection and control, and other similar areas.
2. Discussion of the Related Art
Referring to FIG. 1, a prior art optical sensor 50 is shown. The optical sensor 50 utilizes optical reflectivity to determine Index of Refraction (IoR). The sensor 50 detects the presence of a sample 40 by using a critical angle to find the sample's Index of Refraction, as shown in Equation 1 below, where n2 is the Index of Refraction of the medium of transmission, n1 is the Index of Refraction of the medium of origin (light transmissive housing 55) and θc is the critical angle.n1=n2 sin θc   (1)
As shown in FIG. 1, the necessary electro-optical components are contained within an encapsulating light transmissive housing 55. Light source 57 emits electromagnetic radiation, or light rays 59, 60, and 61 toward a reflective mirrored surface 62. The light rays 59, 60, 61 then travel through the housing 55 in the direction of sensing surface 64 which forms the interface between the sensor 50 and the sample 40. Thus the sensing surface is in direct contact with the sample.
The plurality of light rays 59, 60, and 61 strike the sensing surface 64 at a range of angles. For angles of incidence smaller than the critical angle 75, a portion of the light is refracted into the sample 40 resulting in an overall loss. This is illustrated by refractive ray 63 which travels into the sample and reflected ray 65 which reflects into the housing 55 at angle 74.
At the critical angle 75, a light ray 60 reflects along the sensing surface 64 at a 90 degree angle of refraction minimizing the overall light loss into sample 40. Thus, a critical angle 75 can be measured as the angle measured between the incident light ray 67 and the normal to the sensing surface 64. For angles of incidence larger than the critical angle 75, such as 76, the incident ray 69 is totally internally reflected within housing 55, with no refracted component, and its full intensity is therefore directed toward photodetector 90. This total internal reflection can only occur when light originates in a medium of a higher Index of Refraction.
It should be noted, however, that the Index of Refraction of the housing material may be lower than the sample 40. In such a configuration, the sensor 50 can be used to render a threshold level of Index of Refraction eliminating a range less than that of the housing material.
For optical radiation, a suitable photodetector 90 is the TSL213, TSL401, and TSL1401 (manufactured by Texas Instruments Inc. Dallas, Tex.), with a linear array of resolution n×1 consisting of n discrete photo sensing areas, or pixels. Light energy striking a pixel generates electron-hole pairs in the region under the pixel. The field generated by the bias on the pixel causes the electrons to collect in the element while the holes are swept into the substrate. Each sensing area in the photodetector 90 thereby produces a signal on an output with a voltage that is proportional to the intensity of the radiation (60, 65, 70) striking the photodetector 90. This intensity and its corresponding voltage are at their maxima in the total internal reflection region.
Various means of photodetection are contemplated including an n×1 cell photodetector such as the TSL213, TSL401, and TSL1401. In addition, a single cell photo resistor, bolometer, positive sensitive detector, pyrolectric device as well as other devices may be used.
As described, a range of angles of the reflected light rays are projected onto photodetector 90. The critical angle is marked by a transition from high to low intensity. The output, representing bit level data from the photodetector 90, is transmitted within housing 55 via interface 92 to a signal processing unit 95 for further qualitative and/or quantitative analysis.
The signal processing unit 95 may provide the necessary interface control signals, such as clock line and serial input, for protocol communications with the photodetector 90. Signal processing unit 95 may be used increasing the sensor's resolution over that obtained by the photodetector 90 pixel resolution. The use of the signal processing unit 95 is optional.
When used, signal processing unit 95 is preprogrammed to analyze and characterize the intensity, occurrence, and timing of light rays 60, 65, and 70 to obtain qualitative and quantitative information about the sample 40. For example, signal processing unit 95 can be preprogrammed to determine the total amount of time that sample 40 is within a given proximity of the sensor 50. Also, signal processing unit 95 can be preprogrammed to determine the frequency of sample 40 over a given period of time.
Output data from signal processing unit 95 may be transmitted via interface 96 to a secondary system, such as a computer, hand-held meter, calculator, printer, logic analyzer, or other similar system (not shown). The interface 96 comprises a plurality of conductive pins, giving the sensor a platform similar to an integrated circuit package.
Sensor 50 may also include a temperature sensor 98 within the housing 55. Temperature sensor 98 produces an electrical signal indicative of the temperature of the sensor surface 64 during operation thereof. This temperature signal may be relayed to signal processing unit 90 via interface 97. Temperature data can be utilized to compensate for apparent changes in the measured Index of Refraction as a result of changes in the operating temperature.
A filter may be used to screen out radiation at wavelengths other than wavelengths produced by light source 57. This filter (not shown) may overly photodetector 90 and serves to pass radiation at the wavelengths produced by light source 57 to photodetector 90. As such, the filter eliminates unwanted noise caused by other radiation sources in proximity to the sensor 50. One suitable filter is the plastic filter material marketed by Polaroid Corporation and known as XR-84. This material is especially suitable for passing infrared radiation and blocking visible radiation.
An alternative to utilizing a filter is to utilize a plastic or epoxy material for the housing 50 which is transparent to wavelengths produced by the light source 57 and opaque to frequencies outside the desired frequency range of interest for a given sensor/sample combination. Likewise, an absorbing die can be enclosed in the housing to achieve the same function.
Referring to FIG. 2, a prior art optical sensor 100 is shown. This prior art sensor can be seen to incorporate the same filter features discussed above. The electro-optic components of the optical sensor 100, including light emitting diode 120, photodetector array 110, and temperature sensor 125, may be encapsulated within a trapezoidal-shaped optical housing 150 and coupled to an interior surface 161 of a substrate 160. A plurality of conductive leads 165 are coupled to an exterior surface 162 of the substrate 160. A memory chip 163 is included. An optical window 140 made from glass is attached to the optical housing 150 to form part of sensing surface 145.